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Building Marshall Lead 100 MOSFET
Building Marshall Lead 100 MOSFET
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Old 7th January 2018, 12:03 PM   #11
Mooly is offline Mooly  United Kingdom
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Building Marshall Lead 100 MOSFET
Yes, they would be OK.
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Old 8th January 2018, 09:49 PM   #12
jrow is offline jrow
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And what about capacitors? Can someone tell me a program that calculates voltage by putting schematics?

Thank you.
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Old 8th January 2018, 10:25 PM   #13
GeorgK is offline GeorgK  Austria
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With caps of rather small value like here, there is no need to select voltage just as low as possible. They are neither big nor expensive, also not good quality ones.
Simply choose 63V, this should be safe for all electrolytics. For C42 I would take 160V or similar. C44 could have less, but why?
If you build it, maybe also consider adding an adjustable bias (by R54). For best safety, I would put in a trimmer first, measure its value after adjustment and replace it with a fixed resistor then.

Last edited by GeorgK; 8th January 2018 at 10:30 PM.
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Old 9th January 2018, 10:48 AM   #14
Mooly is offline Mooly  United Kingdom
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Building Marshall Lead 100 MOSFET
Quote:
Originally Posted by jrow View Post
And what about capacitors? Can someone tell me a program that calculates voltage by putting schematics?

Thank you.
You could simulate a circuit like this using something like LTSpice. Click my signature line to see how its done
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Old 9th January 2018, 10:17 PM   #15
jrow is offline jrow
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Great, thank you so much. I'll try to learn LTSpice.

I'll post pics with the result when ready!!

Nevertheless, I'm still thinking about looking for the k134 and j49. I'll see how my wallet and patience is.
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Old 1st October 2018, 02:42 PM   #16
jrow is offline jrow
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Hi!!

I've built the amp. This is how it looks like.


The photos are a bit old so they don't show the power transistors connected. Obviously they are now.

Okay, the amp works. It sounds pretty good and distortion, when the preamp volume is high, sounds incredible. Specially in low string riffs. It's... so natural.

But... volume is very low. I mean, to be a 100 watt amp, when volume is max, sounds like a 20-30 watt amp. In fact, I have a cheap 20 watt amp that sounds a little bit less in the same cabinet. And the fact is that the Mosfet amp sounds with a lot of distortion when volume is high. I don't want always that.

Also, when playing a big time, BUZ transistors don't get too hot. I think that's normal, or at least it was in the original 3210 head.


The amp doesn't make weird noises, and the sound is nice. But the volume... not for gigging.

So, as you told me, by R54 i could set bias. The fact is... I don't know how in this amp. In other threads they don't explain so well.

Please tell me the measurements I have to do in board and, if I put a trimmer in R54, I might start in 100ohm as in the original, and increase, or decrease, or what.

Thank you so much!!


(I used a 2x12 cab with 70watt celestion vintage v-type speakers, connected in 8 ohm config.)
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Old 1st October 2018, 05:25 PM   #17
Mooly is offline Mooly  United Kingdom
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Building Marshall Lead 100 MOSFET
You set the bias by measuring the DC voltage across either of those 0.33 ohm resistors and using ohms law to derive a result.

The correct bias would be 0.1 amp for a single pair of lateral FET's but its not very critical. This means you should see 33mv (0.033 volt) across either of those resistors.

Some practical points...

1/ Ideally you should measure this with no speaker attached, however if the DC offset of the amp is very close to zero then having a speaker attached or not will not influence the result.

2/ The lower the value of R54 and the lower the current will be.

3/ I notice that the design does not have Zobel network on the speaker output which is normally required for total stability. Without using a scope to see what is happening its impossible to tell if there is a problem here or not but instability could appear as distortion.

4/ A single pair of FET's should be good for 70/80 wrms into 8 ohms. If it doesn't sound 'loud' before it all starts falling to pieces then I suspect something else is going on.
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Old 1st October 2018, 08:59 PM   #18
jrow is offline jrow
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The voltage in output is 0.1V. Is it normal, isn't it?

So I will measure what you tell me, Mooly, but it seems biasing won't change the loudness so drastically.

Thank you.
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Old 2nd October 2018, 03:32 AM   #19
JMFahey is online now JMFahey  Argentina
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Please measure power output, ear reckoning of "loudness" isn´t very accurate.

1) confirm you have +/- 47V rails or are real close.

2) check that dummy load is actually 8 ohms

3) scope output to *see* clipping, which is more accurate than hearing it.
You will also see oscillation, unsymmetrical waveforms, gross crossove distortion, etc.

4) set a generator to 1 or 2V RMS or use some App to generate 440Hz or 1kHz, but Phone/Notebook/MP3 player headphone out usually supplies between 100mV to 200mV so you´ll need som kind of preamp.

5) put a 10k to 50k volume pot at the power amp input, set to 0.
Inject 1 to 2 V RS signal (check its value) to pot input, connect wiper to amp, start rising volume slowly while you check the scope screen.

6) once you start seeing clipping, back volume down a little so you barely reach it.

7) only now measure output V RMS and calculate power ... what do you get?

I´d expect at least 80/90 W RMS.

If you get 70W or even way less, say 10 to 30W, then there´s something WRONG which needs troubleshooting.

To begin with, rails will drop something, maybe down to 40V or so, that´s normal.
If they drop to, say, +/- 36/38V or less, your supply is quite inadequate for that amp.

But in any case, only testing and measuring will lead to an answer, just guessing is frustrating.

Well built, that power amp is good.

Which Mosfets did you use?
Where did you get them from?
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Old 2nd October 2018, 07:14 AM   #20
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by jrow View Post
The voltage in output is 0.1V. Is it normal, isn't it?

So I will measure what you tell me, Mooly, but it seems biasing won't change the loudness so drastically.

Thank you.
0.1 volts DC offset will be normal for that design (it is designed without attention to good DC balance) but having an offset like that means that you must measure the bias with no load attached. This is because the 0.1 volts will 'pull' or 'push' a DC current through the speaker, and that current will add or subtract from the small voltage across the 0.33 ohm resistor.

Biasing has zero effect on loudness.
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